Bird box on line (7)
I believe the answer is:
sparrow
'bird' is the definition.
(sparrow is a kind of bird)
'box on line' is the wordplay.
'box' becomes 'spar' (spar can mean to box).
'on' is a charade indicator (letters next to each other) (in a down clue, letters appear on others).
'line' becomes 'row' (a row or line of things).
'spar'+'row'='SPARROW'
(Other definitions for sparrow that I've seen before include "House bird" , "Pirate" , "one on the wing" , "A small chirpy city bird 97)" , "bird produced" .)